Profit Pointer for Improving Changeover Times
Reducing tooling and equipment changeover times is critical to operating a cost-effective tube or pipe mill. Employing quick-change equipment and procedures is the best method for accomplishing this goal. Unfortunately, many tube and pipe producers do not know what type of quick-change equipment is best suited for their mills, the changeover time requirements associated with a particular design, or the cost implications from its use.
Due to the number of variables associated with tube and pipe production, it is impossible to define specific requirements for the application of quick-change equipment; however, general application guidelines can be established. These guidelines relate changeover equipment costs and time requirements to general categories of tube and pipe mills. It is important to remember though, that before any specific equipment is selected, the following parameters must be taken into account: (1) the mill pass arrangement, (2) product specifications, (3) manpower availability and (4) plant capabilities.
Changeover frequency is the number one variable affecting the selection of quick-change equipment. On one hand, a pipe manufacturer might changeover every three or four weeks running the same size product for months at a time. Justifying the cost of quick-change equipment is very difficult. At the other extreme, a tube manufacturer operating in a Just in Time (JIT) or Zero Inventory environment might be required to changeover every shift. If the plant is running three shifts per day, that's fifteen changeovers per week. The cost justification of advanced quick-change equipment is relatively easy. The changeover requirements for 80 percent of tube and pipe producers, all between these two extremes. It is difficult for these manufacturers to select the proper quick-change equipment for their mills and even more difficult to justify the cost.
Roll-Kraft has developed general guidelines for selecting quick-change equipment based on (1) Mill Group (the size range of a particular mill), (2) Changeover time, (3) Tooling Setting and Thread time and (4) Mill line cost for five different categories of quick-change equipment. The four mill groups are listed in Table 1 and the five basic categories of quick-change equipment are listed in Table 2 (Note: Variations in quick-change equipment will exist based on customer and supplier requirements.
Category 1: Basic Mill
This category is non-automated and has one set of mechanicals that is not removed from the main mill during changeover (see Figure 1). The use of an overhear crane and or forklift trucks may be required. Very few Basic Mill Lines have been sold in North America over the last several years. The average changeover time for each Mill Group is listed in Table 3.
Category 2: Basic Rafted Mill
This category of quick-change equipment is the simplest in design and the least expensive (see Figure 2). It typically consists of two sets of mill mechanicals, less the entry tables and scarfing units, mounted on sub-bases or rafts. The use of an overhead crane or forklift truck is required to remove and replace the rafted stands on the main mill bases. The simplest variation to this type of equipment is fitted with manual connections for universal joints, coolant lines and hold down devices. The average changeover time for each Mill Group is listed in Table 4. Rafted mills can be purchased with one set of mill mechanicals and the option to add a second set at a future date. This is a common practice since the cost of a Basic Rafted Mill with one set of mechanicals is approximately the same as the cost of a Basic Mill.
Category 3: Intermediate Quick-Change Mill
This category of quick-change equipment has all the elements of the Category 2 Basic Rafted Mill, plus additional equipment to remove and store the rafted mill bases (see Figure 3). While the equipment changeover requires some manual manipulation by the work crew, the use of an overhead crane or forklift truck is not required. This results in reduced changeover times. Additionally, this type of equipment is fitted with automatic connections for universal joints, coolant lines and hold down devices. The average changeover time for each Mill Group is listed in Table 5.
Category 4: Advanced Quick-Change Mill
This category of quick-change equipment has all the elements of the Category 3 Quick-Change Mill, plus complete automation of all changeover and tooling settings (see Figure 4). The major difference between these two categories of equipment is the fitting of PLC controls to the Category 4 mill. While these controls require a large initial investment, the cost is recouped in changeover times that are significantly less than other types of mills. With PLC controls, an advanced quick-change mill can be changed over with the touch of a button. The only talk required of the work crew are stripping the mill prior to the changeover and threading the mill, with minimal tooling adjustments, after the changeover is completed. The average changeover time for each Mill Group is listed on Table 6.
Category 5: Advanced Quick-Change Mill w/Pre-Threading
This category of quick-change equipment represents the ultimate in quick-change equipment design (see Figure 5). The unique features that distinguish the Category 5 mill from the Category 4 mill is the strip threading equipment. This innovation can thread the strip into the stands while the stands are being changed over. Also, tooling settings and partial threading can be done off-line in the changeover position. Some of the drawbacks to this design are (1) the need for an off-line threading drive, (2) a method to insert the strip into the mill and (3) no reduction in changeover time compared to the Category 4 mill. The minor reduction in threading times is outweighed by the cost and complex logistics, prohibited the use of this equipment in actual practice. The average changeover time for each Mill group is listed in Table 7.
Other Considerations for Improving Changeover Times:
The design and use of quick-change equipment is not the only parameter that affects changeover times. While the equipment plays a dominant roll, other factors come into play. One of the most important of these factors is preventive maintenance, specifically cleaning. Not only must mill equipment be kept clean in order to operate at peak efficiency, but cleanliness is critical for trouble-free changeovers. When bases and stands are exchanged, the replacement units must be accurately aligned in the proper locations. Dirt, grease and mill scale build-up around locating pins and keys can prevent proper alignment. In addition, this dirt build-up can lead to machine jamming. The net result is increased changeover time and machine repair downtime. Properly designed quick-change equipment is fitted with a wash down area. Each time a set of stands comes off the mill, and before the tooling change begins, the set of stands are thoroughly washed and then lubricated. On any Category 3 or higher changeover system, a minimum of two hours per week should be spent on preventive maintenance for the changeover equipment. Another parameter affecting changeover time is the removal and replacement of the tooling on the mill mechanicals after the tooling has been set in the changeover position. Up to this point, the discussion has focused on exchanging mill mechanicals only because this procedure directly impacts mill run time availability. While the mill line on any Category 2 through 5 can produce tube while the rolls are being exchanged, the off-line set-up must be complete and ready to go before the line finishes the size being run. In most cases, this is not a problem, however, on short runs or partial coil runs, production can be completed before the off line tooling change is completed. This will add to the overall changeover time. Also, some additional off line apparatus changes might be required. These include exchanging stand pullers and roll manipulators. In a few cases in which a large number of short runs are the rule rather than the exception, the need for any quick-change equipment is negated.
Upgrading Existing Mills to Improve Changeover Times
Any existing basic mill can be upgraded to any of the Category 2 through 5 mills (see Figure 6). The cost of this upgrade, though, will often exceed that of a new mill due to engineering costs and modifications to existing equipments. This extra cost can be minimized, however, by planning for a step-by-step future upgrade at the time the new mill is ordered. If properly planned the cost of the upgrade will be no more than if the new mill was purchased with the quick-change equipment installed. The questions that the tube and pipe producer must ask are: "Where do I want be to now?" and "Where do I want to be in the future?"